This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Our long-time goal is to understand von Willebrand factor (VWF) assembly into disulfide- linked multimers, tubular packing into secretory vesicles, and secretion into the blood. VWF is an enormous multimeric glycoprotein that mediates platelet adhesion at sites of vascular injury and thus plays a critical role in hemostasis. VWF function depends on assembly into multimers that can exceed 20 million Da, and defects in these large multimers cause a bleeding disorder called von Willebrand disease (VWD). Conversely, the presence of 'ultralarge'VWF multimers in the blood is associated with thrombotic thrombocytopenic purpura, an often fatal disorder characterized by extensive microvascular thrombosis. Thus understanding VWF multimer assembly has substantial medical importance n a previous study, we found that recombinant N-terminal VWF D'D3 disulfidelinked homodimer and the propeptide (domains D1D2) form a Ca2+-dependent noncovalent 280 kDa complex under conditions of neutral pH similar to the ER, and further assemble into massive hollow tubules with up to 30 helical turns and several hundred nm in length at pH values similar to the Golgi. Quick-freeze deep-etch (QFDE) EM and three-dimensional reconstruction of negatively stained images (by Ying Wang in Egelman's lab) showed that the tubules are assembled from a repeating unit of one D'D3 dimer and two propeptides arranged in a right-handed helix with 4.2 units per 11 nm turn. The dimensions of the tubule (outside diameter 25 nm, inside diameter 12 nm) are similar to those of VWF tubules in WPBs seen in thin section of endothelial cells by transmission electron microscopy. We propose to reconstruct intermediate complexes and tubules made in vitro from progressively larger fragments of VWF, and to increase the resolution of the 3D reconstructions by employing cryo EM.